Commercial heating repair. No heat on the rooftop HVAC unit. The pressure switch tubing rusted off the heat exchanger face plate. Connect the new copper tubing and sealer to the faceplate. Unit started up. Inspected the heat exchanger and found heavy rust buildup. No warranty on repair. Will contact the factory about the replacement unit. No heat on the Electric air handler. Replaced the 24-volt heat relay inside the air handler that controls the electric duct heater. Cycled and tested.